The present invention relates to corrosion protection for hollow vessels, and more particularly to corrosion, chemical, and fuel resistance in hollow vessels, such as fuel tanks, used with motorcycles.
A motorcycle includes a frame and an internal-combustion engine coupled to the frame and operable to propel the motorcycle. The internal-combustion engine runs on fuel supplied from a fuel tank that is also coupled to the frame. The fuel tank is a hollow vessel that includes a wall defining a cavity for storing fuel. The wall includes an interior surface exposed to the fuel and an exterior surface exposed to the environment. The fuel tank is commonly fabricated by welding a series of formed patterns of sheet metal. The interior and exterior surfaces usually include enhanced surface treatments such that the interior surface resists the corrosive effects caused by the fuel and the exterior surface resists the corrosive effects of the environment to maintain an aesthetically pleasing appearance.
The enhancement of the interior and exterior surfaces of the fuel tank is typically a two-step process. First, the corrosion resistance of the interior of the tank is addressed. There are many different ways to accomplish the desired corrosion resistance properties. One way is to fabricate the entire tank with stainless steel or nickel plated steel. Although these materials may provide acceptable corrosion resistance, they are significantly more expensive than more commonly-used materials, such as low-carbon steel. A more common way to enhance the corrosion resistance of the interior surface is to use a low-cost, low-carbon steel and to powder coat the interior surface with an epoxy material.
The powder coat process includes first cleaning the cavity, and then applying a dry powder to the clean interior surface. The powder is applied using an electrostatic process where the tank is grounded and the cavity is sprayed with the powder, which consists of charged, non-conducting powder particles. The charged particles are attracted to the interior surface and cling to it. The tank is heated in an oven to fuse the particles into a smooth continuous film. The fused epoxy coating improves the corrosion resistance of the interior surface of the tank.
After the interior surface of the tank is coated, the corrosion resistance of the exterior surface is addressed. One common way to improve the properties of the exterior surface of the tank is through an electrodeposition or electrocoat (E-coat) process. However, before beginning the E-coat process, the tank must be prepared by the labor intensive process of isolating the interior of the tank to protect the interior of the tank. This is done by blocking all of the holes that allow access to the cavity, for example, by using tooling, plugs, and seals to block the tank's fuel fill opening, vent holes, and fuel line ports. Despite this blocking operation, the fuel tanks are susceptible to possible leaking during the subsequent processes, which may cause defects in the cavity and increase scrap costs.
Once the cavity is isolated, the exterior surface of the tank is washed by immersing the tank into a bath or by spraying the exterior surface with cleansers to remove soils, oil, grease, lubricants, and rust. After the exterior surface is cleaned, a conversion coating is applied to the surface to enhance the adhesion of the subsequent E-coat. The conversion coating is typically a phosphate coating (e.g., iron, zinc, or manganese) and is applied to the tank by immersing the clean tank into a hot processing solution for a period of time dependent upon the bath chemistry and material being used.
After the pretreatment with the phosphate conversion coating, the E-coat process is used. E-coat deposition is a process in which positively charged particles are deposited out of a water suspension to coat a conductive part. First the tank is grounded or charged and submerged into a coating tank to begin the cathodic electromechanical process. In this process, the electrically-charged coating binder, pigment, and additives migrate through the water under the influence of an electric field onto the exterior surface. The electrical charge seeks out the path of least resistance and coats the portions of the exterior surface of the tank. As the process continues, the charged particles resume their search for uncoated portions of the tank and begin coating areas that are not as easily reached. The ability to coat these hard-to-reach areas of the tank is known as the coating's “throw-power”.
Once on the part, the charged materials give up their charge due to neutralization by electrochemically generated OH− ions. Upon giving up their charge, the coating materials drop out of the water suspension and coalesce as a coating on the exterior surface. The fuel tank is then removed from the coating tank, rinsed, and cured in an oven, after which, the plugs and seals are removed from the openings in the fuel tank.
In cases where the plugs and seals fail to adequately isolate the cavity, the cavity becomes at least partially filled with the liquid coating of the E-coat process when the fuel tank is submerged in the coating tank. The presence of the liquid coating in the cavity during the curing process damages the integrity of the powder coat on the interior surfaces. Therefore, the “leakers” are typically scrapped or stripped and completely recoated.